A digital amplifier generally include a pulse modulation unit, a switching unit, and a low-pass filter unit. The pulse modulation unit modulates a pulse signal in response to an input analog signal such that a ratio of the “0”-level period and the “1”-level period of the output pulse signal is responsive to the voltage level of the input analog signal. The switching unit switches transistors in response to the output pulse signal of the pulse modulation unit to generate a power-amplified pulse signal. The low-pass filter unit performs low-pass filtering with respect to the amplified pulse signal to generate an amplified analog signal corresponding to the input analog signal.
Transistor elements in digital amplifiers are used in the fully turned-on state or in the fully turned-off state, which provides a higher power utilization efficiency than in the case of an analog amplifier which mainly uses the linear region of transistor elements. Digital amplifiers, however, have a problem of a slow rising response and a slow falling response due to the existence of the low-pass filter. Making higher the cutoff frequency of the low-pass filter for the purpose of achieving a faster rising response and a faster falling response will result in a noise increase. Because of such a tradeoff between response time and noise, it is difficult to increase response speed when taking into account the existence of noise.
The use of an analog amplifier can achieve a fast rising response and a fast falling response. The use of an analog amplifier, however, entails a low amplification efficiency, resulting in electric power being wasted.
U.S. Pat. No. 5,889,392 discloses a technology for addressing the problem that a DC-DC converter using a switching regulator cannot cope with a sudden change in load current because of the existence of the low-pass filter. This technology uses an additional circuit for speeding up an output-voltage transient response in the configuration in which the output voltage of an amplifier is fed back to allow the switching regulator to control the output voltage such that the output voltage is set equal to a desired reference voltage VREF. This speeding-up-purpose circuit includes a transistor to pull up the output voltage toward the power-supply voltage level and a transistor to pull down the output voltage toward the ground voltage level. The output voltage deviating from VREF by more than ±2% results in a transistor being turned on, thereby swiftly bringing back the output voltage within a range of ±2-% around VREF.
The circuit configuration disclosed in the above-noted U.S. patent drives the speeding-up-purpose circuit in response to the detected value of the output voltage to cause the output voltage to rapidly change. However, in the digital amplifier whose input voltage and output voltage are supposed to change, it is not appropriate to drive a speeding-up-circuit based on the detected value of the output voltage. Further, in the case that the constantly changing input voltage causes the output voltage to always fall outside the range of ±2-% around VREF, the speeding-up-purpose circuit ends up operating all the time, resulting in a poor efficiency.